Some electronics devices are able to operate from either alternating current (AC), also known as line power, or a battery. In such devices, as long as line power is provided, the device operates on line power and recharges the battery. When line power is removed, the device operates on battery power, and the battery discharges until line power is once again provided.
Repeatedly using the battery rather than line power to operate the device reduces the lifetime of the battery, because powering the device from the battery discharges the battery, and battery lifetime is limited to some number of charge/discharge cycles. Sometimes a user removes line power from the device without realizing that doing so will negatively affect the battery lifetime. One example of this situation occurs when line power to the device is used in conjunction with a switch, such as a light switch or the switch on a power strip.
In this configuration, a user may turn off the switch, and thus line power to the device, without realizing the device is affected. During the time that the switch is off, battery-powered devices will deplete their battery charge. This has the effect of reducing battery life if done repeatedly. Another effect is that the device battery will be in a lower state of charge if a true power outage were to occur. This is particularly true for devices which rely on battery operation to deliver a critical service, such as when a device interfaces to the telephone network or an alarm system.